I've always wanted a nice fake SMTP server for testing outbound emails, without actually having to run a full fledged SMTP server and wait for my email to be delivered to test the message.
SMTP Server For Developers is a nifty little program by Antix Software that does just that. You set your SMPT server to be the address of the computer running it and it'll log all SMPT requests.
The requests are stored in an EML file that email clients can open, such as Thunderbird and Outlook.
There is no installation required, so just run it when needed. It needs the .NET framework to be installed (not sure which version), so I presume it'll work with MONO as well.